Adding documentation for new APIs in DecompressionMethods and MultipartContent classes. (#2843)

* Adding documentation for new APIs in DecompressionMethods and MultipartContent classes.

* Apply suggestions from code review

Co-Authored-By: Ron Petrusha <[email protected]>

Author: jozkee

xml/System.Net.Http/MultipartContent.xml

@@ -197,9 +197,20 @@
       </ReturnValue>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
-        <remarks>To be added.</remarks>
+        <summary>Serializes the HTTP content to a stream using the multipart/* encoding as an asynchronous operation.</summary>
+        <returns>The task object representing the asynchronous operation.</returns>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+ This method overrides <xref:System.Net.Http.HttpContent.CreateContentReadStreamAsync%2A?displayProperty=nameWithType> to use a custom stream that contains an array, with each HTTP content and its boundary encoded and serialized to a <xref:System.IO.MemoryStream> instance.
+
+ This operation will not block. The returned <xref:System.Threading.Tasks.Task%601> object will complete after all of the content has been written to the memory stream.  
+  
+ Once the operation completes, the <xref:System.Threading.Tasks.Task%601.Result%2A?displayProperty=nameWithType> property on the returned task object contains the stream that represents the multipart/* encoded HTTP content. The returned stream can then be used to read the content using various stream APIs.  
+  
+ ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="Dispose">
@@ -411,4 +422,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>

xml/System.Net/DecompressionMethods.xml

@@ -59,7 +59,7 @@
       </ReturnValue>
       <MemberValue>-1</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Use all compression-decompression algorithms.</summary>
       </Docs>
     </Member>
     <Member MemberName="Brotli">
@@ -85,7 +85,7 @@
       </ReturnValue>
       <MemberValue>4</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Use the Brotli compression-decompression algorithm.</summary>
       </Docs>
     </Member>
     <Member MemberName="Deflate">
@@ -191,4 +191,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>